1. Field of the Invention:
This invention relates to a method for the production of maleimides. More particularly, this invention relates to a method for the production of maleimides by ring-closure imidation of maleinamic acids.
2. Description of the Prior Art:
Maleimides are compounds useful as raw materials for synthetic resins, medicines, and agricultural chemicals. Researches after methods for their produciton have long been under way. The most popular method of them all effects the production of maleimides by the dehydration cyclization of maleinamic acids with a dehydrating agent such as acetic anhydride. One version of this method is disclosed in U.S. Pat. No. 2,444,536. This method effects the production of maleimides by causing maleic anhydride to react upon amines thereby forming maleinamic acids and dehydration cyclizing and, at the same time, imidating the maleinamic acids in the presence of acetic anhydride and sodium acetate. This method, however, has the disadvantage that the imidation requires expensive acetic anhydride to be used in at least an equivalent relative to the maleinamic acid and the separation and recovery of the formed maleimide from the imidation reaction solution necessitates use of a large volume of water and, as the result, entails disposal of a large amount of an acetic acid-containing effluent at great expense. Thus, this method may will be called a too expensive method for commercial production of maleimdies.
A method which has no use for such a chemical dehydration agent as acetic anhydride is disclosed in British Pat. No. 1,041,027 and U.S. Pat. No. 3,431,276. This method effects the production of maleimides by thermally dehydrating and cyclizing maleinamic acids in conjunction with a solvent such as, for example, toluene, xylene, or chlorobenzene having a boiling point exceeding 80.degree. C. and serving as a diluent and an acid catalyst such as sulfur trioxide, sulfuric acid, or ortho-phosphoric acid, and distilling the system thereby causing azeotropic expulsion of the consequently formed water in conjunction with the solvent. As compared with the method which uses acetic anhdyride, this method proves advantageous in that it does not require use of a large amount of such an expensive dehydrating agent as acetic anhydride and further that the formed maleimides are separated and recovered with ease. This method nevertheless has the disadvantage that the yield of the imidation is low as compared with that obtainable by the method using acetic anhdyride. This disadvantage is logically explained by a postulate that compared with the method which effects the imidation by the use of acetic anhydride, the method which effects the imidation by performing thermal dehydration in the specific solvent as described above involves a high reaction temperature and, therefore, tends to induce side reactions and inevitably manages to produce maleimides abounding with impurities and further that since maleimdies are thermally unstable, the maleimides produced at all are degenerated during the course of the reaction. Further, as a commercial process, this method is not ecconomically satisfactory, because it requires to use an expensive acid catalyst in a relatively large amount and, moreover, produces the maleimides in a low yield.
There is another method which, as disclosed in Japanese Patent Laid-Open No. SHO 53(1978)-68,700 and Japanese Patent Publication No. SHO 57(1978)-42,043, comprises causing maleic anhydride to react on amines in the presence of an organic solvent thereby forming maleinamic acids and subjecting the maleinamic acids as held in a state not isolated from the reaction system to dehydration and cyclization in the presence of such an aprotic polar solvent as dimethyl formamide or dimethyl sulfoxide and an acid catalyst. By this method, there is offered recognizable improvement in yield as compared with the second method described above. This method, however, has these disadvantages, that the cost of production of maleimides is high because expensive and highly toxic aprotic polar solvent such as dimethyl formamide is used in a large amount, that the solvent such as dimethyl formamide is degenerated by the action of an acid catalyst used in the reaction and, therefore, the solvent is lost greatly, and that since the aprotic polar solvent used in the reaction has a high boiling point, the solvent is removed from the produced malimides with great difficulty.
Japanese Patent Laid-Open No. SHO 54(1979)-30,155 discloses a method for producing an oligoimide by using, as a catalyst, a mixture of an inorganic or organic acid-containing acid with a quaternary ammonium salt of the acid. The quaternary ammonium salt which is used as mixed with an acid catalyst in this method, however, is an ammonium salt of the nitrogen atom of which has been at least disubstituted. Specifically, this is an expensive interphase catalyst such as dimethyldialkyl ammonium methane sulfonate or tetraoctyl ammonium methane sulfonate. The method, thus necessitating use of such a compound as indicated aobve, is inevitably judged to be an expensive approach. For this method to maintain a highly satisfactory yield of imidation, however, it is essential that the reaction should be continued with the ratio of the acid catalyst to the quaternary ammonium salt rigidly controlled within a certain range. When the catalyst which has been used once in the reaction is used again, the imidation cannot be obtained in a highly satisfactory yield because the ratio is varied in the presence of the used catalyst. An effort to attain efficient reuse of the used catalyst, therefore, entails as a problem the fact that the management for maintenance of catalytic activity as by subjecting the used catalyst to purifying and readjusting treatment calls for immense labor.
Japanese Patent Laid-Open No. SHO 60(1985)-109,562 discloses a method for the production of monomaleimide by the cyclizing imidation of maleinamic acid in a mixed solvent containing a nonpolar solvent such as toluene or xylene and a polar solvent such as dimethyl sulfoxide or N-methyl pyrrolidone in a specific ratio in the presence of an acid catalyst such as p-toluenesulfonic acid or m-toluenesulfonic acid and a mixed catalyst containing the acid catalyst and an ammonium salt such as, for example, the salt thereof with maleinamic acid. In this method, however, since the acid catalyst and the polar solvent in the mixed solvent react with each other to form a complicate complex (which is widely variable with the ratio of the amounts of the two compounds and the temperature, for example), it is the complicate catalyst system composed of the complex just mentioned, the acid, and the salt that substantially produces a catalytic activity. Thus, the yield of the imidation is affected to a great extent by the composition of the three components mentioned above. When the reaction is carried out batchwise, it does not intail any appreciable disadvantage. When the reaction is carried out in such operation system as require the catalyst and the solvent to be used in a recycling manner, however, it entails various drawbacks. To be specific, this method renders the selection of reaction conditions complicate because the amount of the complicate complex produced owing to the use of the polar solvent is varied and the catalyst is varied in quality from one batch to another. This is equivalent to a statement that the method under discussion has the disadvantage that it is unfit for a continuous reaction.
An object of this invention is to provide an improved method for the production of maleimides.
Another object of this invention is to provide a method for producing a maleimide of high purity in a high yield by a safe and simple procedure.
Yet another object of this invention is to provide a method for enabling the production of a maleimide to be effected easily by a continuous reaction.